This invention relates to a solid electrolytic capacitor and a method of making it and, more particularly, to a solid electrolytic capacitor and a method of making such capacitor in which an electrically conductive polymeric compound is employed as the solid electrolytic material.
In general, a solid electrolytic capacitor includes a dielectric oxide film disposed on a surface of a valve metal, one or more layers of solid electrolyte disposed on the dielectric oxide film, and a carbon layer and a silver layer disposed on the solid electrolytic layers. An anode lead is connected to the porous anode body, while a cathode lead is connected to the silver layer. After that, an outer coating of resin material is provided over the outer surface.
Known electrically conductive polymeric materials useable as a solid electrolyte of the solid electrolytic capacitor include polyacetylene, polypyrrole, polyaniline, polythiophene and poly-para-phenylene. In particular, polypyrrole, polyaniline and polythiophene are more frequently used because they have good conductivity and good thermal stability.
Japanese Unexamined Patent Publication (KOKAI) No. HEI 4-48710, for example, discloses a solid electrolytic capacitor having a solid electrolyte including two layers of electrically conductive polymer, which are formed by first disposing, on a dielectric oxide film, an electrically conductive polymer layer of polypyrrole by chemical polymerization, and, then, disposing, on the polypyrrole layer, another conductive polymer layer of polypyrrole by electrolytic polymerization. It is, however, difficult to form a uniform polypyrrole layer by chemical polymerization, and a polypyrrole layer can hardly be disposed on minute structures, such as recesses in a sintered body and etching pits. Thus, resulting products disadvantageously exhibit low capacitance and high impedance.
No. HEI 3-35516 shows solid electrolytic capacitors having a solid electrolyte including a film of polyaniline formed by applying a solution of pre-polymerized polyaniline over a surface of a dielectric film and drying it. Since the polyaniline solution has a high viscosity, it hardly enters into recesses in a sintered body formed of minute powder particles of tantalum or recesses in an oxide film on an aluminum foil. Accordingly, this method can produce capacitors disadvantageously having very small capacitance. It may be possible to form polyaniline by polymerizing aniline monomer on the oxide film. Capacitors produced by this method would satisfy the requirements on capacitance, but, because the conductivity of polyaniline itself is lower than polypyrrole, the impedance characteristic of the resulting capacitors exhibited in a high frequency region is not so good as capacitors with a polypyrrole layer.
In order to solve the above-described problems, Japanese Unexamined Patent Publication (KOKAI) No. HEI 9-320900 discloses a capacitor with a solid electrolyte of polyethylenedioxythiophene (hereinafter referred to as PEDT), which can be more easily formed into a uniform layer than polypyrrole and polyaniline. While PEDT can be easily formed into a uniform layer, a resulting layer is so thin that it is necessary to repeat a polymerizing process several tens of times in order to obtain a solid electrolyte extending to an outer periphery of a capacitor, which makes the manufacturing process very troublesome. If the number of the times of repetition of polymerization process is reduced, leakage current from the resulting capacitor is too large to provide desired characteristics.
In order to solve the above-discussed various problems, a solid electrolyte including a composite layer of conductive polymers may be used. In such a case, an outermost conductive polymer layer, which is to contact a carbon layer, is, in many cases, a layer formed by electrolytic polymerization. Although a conductive polymer layer formed by electrolytic polymerization can be thick and durable, its surface is flat and, therefore, it exhibits a low mechanical and electrical bonding strength with respect to the carbon layer. Thus, resulting capacitors have disadvantages in their mechanical strength, contact resistance between the conductive polymer layer and the carbon layer, and impedance characteristics in a high frequency region.
A first object of the present invention is to realize a solid electrolytic capacitor having an anode electrode including a sinter of minute powder of a valve metal, which has large capacitance, exhibits good impedance characteristics in high frequency regions, and has a durable, electrically conductive composite polymer layer forming a solid electrolyte layer.
A second object of the present invention is to realize a solid electrolytic capacitor which exhibits high bonding strength between a composite, conductive polymer layer and a carbon layer formed thereon.
A solid electrolytic capacitor according to the present invention includes a composite, electrically conductive polymeric material layer structure including at least three layers disposed on a dielectric oxide film, which, in turn, is disposed on a surface of an anode body of a valve metal, e.g. tantalum. A first layer contacting the dielectric oxide film is formed of polythiophene or its derivative.
A second layer is an electrically conductive polymer layer formed by chemically polymerizing a material different from that of the first layer. As occasion demands, a third layer of polythiophene or its derivative may be disposed on the second layer. A fourth layer of electrically conductive polymer is formed by electrolytic polymerization on the third layer or on the second layer in the absence of the third layer.
Polythiophene or its derivative forming the first layer has advantages that it has good electrical conductivity, can be made to enter into the interiors of minute pores in the sintered anode body when an appropriate processing is employed, and can uniformly coat the surface of the dielectric oxide film. However, it has a disadvantage that the layer formed thereof is thin.
Polypyrrole, polyaniline or a derivative thereof is suitable as the electrically conductive polymer material for the second layer. When it is disposed to overlie the first layer by chemical polymerization, the second conductive polymer layer can be relatively thick and can be intimately bonded to the first conductive polymer layer. The second layer, together with the first layer, and also with the later-mentioned third layer, provides a durable, electrically conductive polymer multi-layered structure.
An electrically conductive polymer layer of polythiophene or its derivative, which provides the third layer, when disposed on the second conductive polymer layer, can provide the resulting multi-layered structure with higher electric conductivity and fineness.
The fourth conductive polymer layer is formed on the second or third layer by electrolytic polymerization so that it can provide sufficient durability and thickness for the composite electrolytic layer-structure of the solid electrolytic capacitor. The above-described conductive polymer multi-layered structure can couple current required for the electrolytic polymerization uniformly to the polymeric material forming the fourth layer.
Thus, since the composite, electrolytic polymer layer structure of the above-described solid electrolytic capacitor can extend into minute pores in the sintered anode body to cover the entire surface of the dielectric oxide layer, it can provide the resulting solid electrolytic capacitor with large capacitance and good impedance characteristic. Furthermore, it can be thick and have good mechanical and thermal strength. A carbon layer is disposed on the composite, conductive polymer layer structure.
According to the present invention, a fifth electrically conductive polymer layer may be additionally formed between the fourth layer and the carbon layer by chemical polymerization. For the fifth layer, polythiophene, polypyrrole, polyaniline or a derivative thereof is a suitable material. The electric conductivity of the fifth layer can be increased by mixing into it, from 1 to 50% by weight of powder of carbon and/or the conductive polymer material same as that of the fifth layer.
Since the fifth layer provides improved bonding between the composite conductive polymer layer and the carbon layer, it contributes to the reduction of the internal resistance component of the capacitor element to thereby improve the impedance characteristic of the capacitor and also increase the mechanical strength of the capacitor.
A suitable thiophene derivative for the conductive polymer layer(s) may be a thiophene derivative which has at least one member selected from a hydrogen group, an acetyl group, a carboxyl group, an alkyl group and an alkoxyl group, as a substituent group, in the 3-position, the 3- and 4-positions or the S-position of the thiophene skeleton, or it may be 3,4-alkylenedioxythiophene.
For the second, conductive polymer layer, pyrrole or its derivative, or aniline or its derivative may be suitable. A suitable pyrrole derivative may have at least one member selected from the group of hydroxyl, acetyl, carboxyl, alkyl and alkoxyl groups as a substituent group at the 3-position, the 3- and 4-positions or the N-position of the pyrrole skeleton. A suitable aniline derivative useable for the second layer may have the aniline skeleton and has at least one member selected from the group of alkyl, phenyl, alkoxyl, ester and thioether groups as a substituent group.
The first, second, third and fifth chemical polymerization layers may be formed by, for example, a xe2x80x9cthree-solution processxe2x80x9d, a xe2x80x9ctwo-solution processxe2x80x9d or a xe2x80x9cone-solution processxe2x80x9d. The three-solution process is a process in which a capacitor element is immersed successively in a monomer solution, a dopant solution and an oxidizing agent solution, and, then, polymerization is caused to take place at a predetermined temperature. The two-solution process is a process in which a capacitor element is immersed in a monomer solution and, thereafter, in a mixture solution of a dopant and an oxidizing agent, or the element is immersed in a mixture solution of a monomer and a dopant and, then, in a mixture solution of a monomer and an oxidizing agent, before the polymerization is caused to take place at a predetermined temperature. The one-solution process is a process in which a capacitor element is immersed in a mixture solution of a monomer, a dopant and an oxidizing agent or in a mixture solution of a monomer and a dopant having oxidizing ability before polymerization is caused to take place at a predetermined temperature. Whichever process is employed, the remaining oxidizing agent must be removed after the polymerization by rinsing the element in water. The film-forming polymerization process may be repeated plural times to produce a layer of a desired thickness.
As for the first and third layers, a polymer layer may be formed by immersing a capacitor element in a solution of polymer of thiophene, its derivative or an intermediate polymer thereof, and, then, heating the assembly.
There is no special requirement for the dopant which can be used in the present invention, but, for providing solid electrolytic capacitors with good characteristics, a sulfonic acid compound may be preferably used. For example, the following may be used: 1,5-naphthalenedisulfonic acid, 1,6-naphthalenedisulfonic acid, 1-octanesulfonic acid, 1-naphthalenesulfonic acid, 2-naphthalenesulfonic acid, 2,6-naphthalenedisulfonic acid, 2,7-naphthalenedisulfonic acid, 2-methyl-5-isopropylbenzenesulfonic acid, 4-octylbenzenesulfonic acid, 4-nitrotoluene-2-sulfonic acid, m-nitrobenzenesulfonic acid, n-octylsulfonic acid, n-butanesulfonic acid, n-hexanesulfonic acid, o-nitrobenzenesulfonic acid, p-ethylbenzenesulfonic acid, p-chlorobenzenesulfonic acid, p-dodecylbenzenesulfonic acid, p-toluenesulfonic acid, p-nitrobenzenesulfonic acid, p-pentylbenzenesulfonic acid, ethanesulfonic acid, camphorsulfonic acid, dinonyinaphthalenesulfonic acid, cetylsulfonic acid, dodecylsulfonic acid, trichlorobenzenesulfonic acid, trifluoromethanesulfonic acid, hydroxybenzenesulfonic acid, butyinaphthalenesulfonic acid, benzenesulfonic acid, polyvinylsulfonic acid and methanesulfonic acid. Also, their salts may be used, which include, for example, lithium salt, potassium salt, sodium salt, silver salt, copper salt, iron salt, aluminum salt, cerium salt, tungsten salt, chromium salt, manganese salt, tin salt, methyl ammonium salt, dimethyl ammonium salt, trimethyl ammonium salt, tetramethyl ammonium salt, ethyl ammonium salt, diethyl ammonium salt, triethyl ammonium salt, tetraethyl ammonium salt, ethyl methyl ammonium salt, diethyl methyl ammonium salt, dimethyl ethyl ammonium, triethyl methyl ammonium salt, trimethyl ethyl ammonium salt, diethyl dimethyl ammonium salt, propyl ammonium salt, dipropyl ammonium salt, isopropyl ammonium salt, di-isopropyl ammonium salt, butyl ammonium salt, dibutyl ammonium salt, methyl propyl ammonium salt, ethyl propyl ammonium salt, methyl isopropyl ammonium salt, ethyl isopropyl ammonium salt, methyl butyl ammonium salt, ethyl butyl ammonium salt, tetramethylol ammonium salt, tetra-n-butyl ammonium salt, tetra-sec-butyl ammonium salt, tetra-t-butyl ammonium salt, piperidinium salt, pyrrolidinium salt, piperazinium salt, pyridinium salt, xcex1-picolinium salt, xcex2-picolinium salt, xcex3-picolinium salt, quinolinium salt, isoquinolinium salt, pyrrolinium salt and ammonium salt.